1. Field of the Invention
The present invention relates to earth boring tools, and more particularly to core catchers as included within a coring tool.
2. Description of the Prior Art
A coring tool is typically comprised of a toroidally shaped coring bit defined by an outer gage and inner gage. The coring bit cuts a cylindrical core from the rock formation, which core is then disposed through its inner gage and generally aligned with the longitudinal axis of the drill string. The coring bit is typically coupled to a bit shank which is coupled to an outer coring barrel and drill collar, and in turn to a drill pipe, which extends to the ground surface and through which drill pipe the rotary motion is transmitted to the coring bit. Still drilling fluid is forced down through the inside of the drill pipe and ultimately the inner gage of the coring bit, a coring barrel typically includes an outer barrel and one or more inner concentric barrels or sleeves aligned with the longitudinal axis of the drill string which separate the core from the drilling fluid. Once the core is cut, particularly in a consolidated formation, some means is required to break or detach the core from the rock formation from which it extends. The prior art has devised a number of different designs for core catchers which are a collection of wedge shaped, curved segments. Segments collectively form a cylindrical split ring having an internal diameter which frictionally engages the outside surfaces of the core as the barrel is moved downwardly over the core. After the core is cut, the core barrel is pulled upwardly. The core catcher seizes the core and is moved downwardly with respect to the core barrel against an internal frustoconical surface on an inner diameter of the coring tool. Thus, the core catcher becomes wedged between the interior surface of the coring tool and the core. Ultimately, this allows the full tension applied to the drill string to be applied to the core. The core is then broken from the rock formation and retained in the core barrel for retrieval and removal at the surface.
However, the use of such a traditional core catcher is entirely ineffectual in fragmented, loose, sandy or otherwise unconsolidated formations. Firstly, many of such cores need not be broken from their underlying rock formations. Secondly, such traditional rigid core catchers are unable to seize upon the core and wedge it within the core barrel with the result that some or all of the core is lost as the coring tool is tripped from the borehole. In order to overcome these difficulties, the prior art has devised a number of full closure catchers which are typically multiple cusped flapper valves installed in the interior of the core barrel in such a manner that the core is allowed to move upwardly within the core barrel, but upon any downward movement of the core, the flapper valves engage the core and are rotated to a shut or fully closed position. This downward movement is typically gravitationally induced. Thus, even for a sand core the relative downward movement of the sand column relative to the flapper valves causes the flapper valves to dig into the sand core and rotate to provide a full closure of the inner diameter of the core barrel.
However, even with such full closure core catchers the placement of the flapper valves within the core barrel space tends to create an obstruction upon which the core may jam as it is being cut; or provides a means which physically disturbs the original stratification in the core. Obstruction or disturbance of the core are further increased by the use of spring biased flapper valves, which are used at times to increase the reliability of the system.
Therefore, what is needed is a full closure core catcher which overcomes each of the defects of the prior art designs, and in particular, a full closure core catcher which presents no obstacles or opportunities to jam or disturb the core as it is being cut and disposed within the core barrel, but which is reliably and securely driven into a closed position once cutting of the core is finished.